According to the US. Food and Nutrition Board, the Recommended Daily Allowance (RDA) of calcium during pregnancy is 1200 mg/day. The RDA for the adolescent during the growing years is also 1200 mg/day. There is no recommendation for an increment in calcium intake to meet the presumably greater calcium needs of pregnant adolescents or for multiple gestation pregnancies: specifically calcium and bone metabolism changes during these higher risk pregnancies, and the effect of calcium intake on maternal calcium and bone metabolism are unclear. Recent studies suggest that conventional dietary calcium intake during pregnancy may be inadequate to maintain a positive calcium balance and optimal bone metabolism, and several studies have also demonstrated a direct relationship between low calcium status and higher incidence of hypertension during pregnancy which may lead to a significant neonatal morbidity. Theoretically, calcium imbalances may occur when there is a need for high fetal calcium accretion, such as in adolescent pregnancy, and also in multiple gestation, because pregnancy occurs during the period of active maternal bone growth. The overall objective of this longitudinal randomized trial is to determine whether there are significant alterations in maternal serum calcium, calcium regulatory hormone, and bone mass indices during adolescent pregnancy and adult multiple gestation, and whether these changes are influenced by dietary calcium intake. The hypotheses to be tested are: l) there is a greater need for calcium and a greater increase in serum calcium regulatory hormone concentration (PTH and calcitriol), among adolescent pregnancies and adult multiple gestations compared to adult singleton pregnancy; and, 2) pregnant adolescents and females with multiple gestation who are randomized to receive calcium supplement (1000 mg/day) will have less of a decrease in serum calcium, and less of an increase in serum PTH and calcitriol throughout pregnancy compared to similar women randomized to placebo. Changes in biochemical indicators of bone metabolism (Osteocalcin, serum carboxyterminal propeptide, and serum cross-linked telopeptide of type I collagen) also will be investigated. The trial will be conducted in 336 pregnant women. Participants will include healthy pregnant females between 21 and 40 years of age, with singleton and multiple gestations, and singleton mothers age 12 to 20 years. This study will aid in the understanding of calcium and bone metabolism and calcium requirements in adolescent pregnancy and adult multiple gestation; and may help in the understanding of the pathophysiology of hypertension during pregnancy.